1. Field of the Invention (Technical Field)
The present invention relates to materials for enhanced band bending, particularly for use in rectifying contacts. These materials comprise metal nanoparticles and a semiconducting polymer.
2. Description of Related Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Metal contact to semiconductor interfaces manifest basic features of many rectifiers and metal semiconductor (MS) devices which are critical elements in a number of important technologies, such as OLEDs, field effect transistors, and sensors. When a metal is making intimate contact with a semiconductor, the Fermi-levels in the two materials must be coincident at thermal equilibrium.
Depending on the difference in work functions of the metal and semiconductor, the contacts may be either ohmic or non-ohmic, the interface of the latter being a rectifying (Schottky) contact. An ohmic device is one that demonstrates Ohm's Law V/I=R where resistance R is a constant, V is voltage and I is the current. In semiconductor devices, highly doped regions are referred to as ohmic contacts and approximate ohmic responses even though moderately doped regions are strongly dependent on voltage.
A Schottky barrier is created by the intimate contact of a metal surface and a semiconductor surface, and depends on the metal's work function, the band gap of the semiconductor, and the type and concentration of dopants in the semiconductor. FIG. 1 shows a Schottky barrier of a metal and a p-type semiconductor. FIG. 1(a) shows the relationship before contact and FIG. 1(b) shows the resulting Schottky barrier after contact.
At equilibrium, in the absence of externally applied voltages, the Fermi level must be constant throughout the sample. Otherwise, a current would flow. In the metal, the Fermi level is the top of the electron sea, while in the semiconductor, far from the interface, the Fermi level is determined by the impurity level. Doping allows manipulation of the Fermi level. When junctions are formed between materials with different Fermi levels, band bending occurs in such a way that the Fermi levels equate across the junction.
The Fermi level is matched as follows. Before equilibrium, the Fermi level is lower in the semiconductor (when the work function of the polymer, Evac−EF=χ+Vn, is larger than that of the metal, φm). Therefore, electrons will flow from the metal to the semiconducting polymer. This causes the build-up of charges on both sides of the interface, resulting in an electric field and therefore a potential gradient according to Poisson's equation d2V/dx2=ρ(x). This is the so-called band bending; different metals result in different levels of band bending in the semiconductor. In this region, the electric field has caused the holes to move away from the interface; they drift to the top of the valence band. The result is that in this area of width W there is a surplus of negative charge caused by uncompensated charged acceptors, and this region is said to be the depletion region because there is an absence of majority carriers (holes in p-type semiconductors).
The parameters that describe the Schottky barrier are the barrier height (φB) the built-in voltage (Vbi) and the depletion width (W). The barrier as seen by (majority) carriers coming from the metal is the barrier height which depends on the difference in electron affinity of the metal and the semiconductor and which (for p-type semiconductors) also depends on the energy gap Eg. The barrier as seen by (majority) carriers going into the metal is the built-in voltage or zero-bias band bending. This is determined by the difference in Fermi level before contact. The depletion width is the width of the area devoid of (majority) carriers.
The transport of current in a metal semiconductor junction is primarily the result of majority carriers. The transport equation of an ideal metal semiconductor rectifying contact is given by the following Schottky barrier diode equation:J=Jo[exp(qV/n kT)−1]where J is the total current density, Jo is the value of the reverse saturation current density, q is the charge of an electron, V is the applied voltage, k is the Boltzmann constant, T is absolute temperature, and n is the diode quality (ideality) factor.
For inorganic semiconductor/metal contacts, an explicit relationship between the barrier height φb and Jo can be obtained from thermionic emission/diffusion theory as:Jo=A**T2[exp(−qφb/kT)]where A** is the modified Richardson constant. For an electron in free space, A=120 A/cm2K2. For high enough voltages (at room temperature), the Schottky barrier diode equation can be simplified by considering the exponential term in the brackets to be dominant. Then, the slope of the logarithmic plot is related to the quality factor n through:
      1    n    =            kT      q        ⁢                            ∂          ln                ⁢                                  ⁢        J                    ∂        V            
Technology is constantly pushing the limits of microelectronics, providing for increasingly smaller and faster circuits. Consequently, current fabrication methods and materials are nearly at their theoretical limits. Nanoscale materials composed of either metal or semiconductor particles are playing an increasingly important role as novel building blocks in physics, solid state chemistry, and materials science. Because of the high surface to volume ratio of nanoparticles, surface properties have significant effects on structural and optical properties. By controlling the size and surface of a nanocrystal, its properties can be tuned.
The literature of the prior art discloses the fabrication of nanocomposites comprising polymers. For example, Curran et al., Adv. Mater. 1998, 10, No. 14, disclose “doping” poly(m-phenylenevinylene-co-2,5-dioctoxy-p- phenylenevinylene) (PmPV) with multiwalled nanotubes to increase electrical conductivity of the polymer. U.S. Pat. No. 6,576,341 discloses using a polymer (PmPv) to purify nanotube “soot” by extracting nanotubes from their accompanying material. U.S. Pat. Application Publication No. 2005/0043503 discloses a polymer-nanotube composite comprising PmPV to enhance such properties as the solubility of solvents. U.S. Patent Application Publication No. 2004/0241900 discloses an organic semiconductor material in which carbon nanotubes are dispersed in a conjugated polymer. Raula et al., Langmuir 2003, 19, 3499-3504, describe monolayer-protected clusters (“MPC's”) of metal nanoparticles having specific electronic, optoelectronic, and catalytic properties. The MPC's comprise a metal core and a shell ranging from small organic compounds to macromolecules and includes gold nanoparticles covalently bound with poly(N-isopropylacrylamide) and with a mixture of poly(N-isopropylacrylamide) and polystyrene. Carroll, SPIE Nanotechnology e-bulletin, June 2004, 1-2, describes advancing organic optoelectronics with charge transfer nanocomposites comprising blends of conjugated polymers and dispersed single-walled carbon nanotubes (SWNTs). Ettedgui et al., Surface and Interface Analysis, Vol. 23, 89-98 (1995), describe band bending in PPV in contact with Al. Band bending occurs, leading to barrier formation which presumably impedes electron flow in polymer-based devices, during the deposition of Al on pristine PPV samples in accordance with the Schottky barrier formation. Bley et al., Rev. Adv. Mater. Sci. 5 (2003) 354-362, describe a method for incorporating SWNTs into PmPV.
At present, the quality and performance of devices made from metal semiconductor contacts is poor because interface states have detrimental effects, causing an unwanted change of band structure and thus blocking possible conduction paths.